I started really using Linux (Red Hat 4.2 maybe?) in about '97, originally I used it for NAT, sharing one internet connection for my many computers on my local LAN. This was back before consumer routers were available.

Soon I started using IPChains, and later IPTables to do custom firewall rules, and allow for very controlled external access to my internal hosts.

At home I now use it for all of that, plus mail server, spam filter (MailScanner, SpamAssassin etc) web server, backups, tons of things. Also a great free open source video surveillance system to spy on my dogs while I'm at work.

At work, I use various security tools like Snort, Arpwatch, and network monitoring and a great helpdesk program. Some of these apps competes with the big multi-thousand dollar commercial apps for Free!

I joined because of the cooperative nature of open-source projects like Linux. I think that this is the way of the future... I see it it the latest versions of Ubuntu which I feel has already eclipsed Windows (yes, even W7) and is rapidly gaining on MacOS which had wisely adopted a UNIX based OS years ago.

People will and must be allowed to modify and copy their programs as they see fit. Linux is the only OS that allows and even encourages such behavior.

I appreciate the selfless talent that goes into any Linux distro, even those I can't comprehend well. As time passed I have seen Linux grow from a command-line oddity to a full-fledged operating system with a top-rate GUI for dummies like me.
In ten years I suspect that Linux in some form will be the dominant OS with MacOS as an alternative. Microsoft's inferior and expensive proprietary OS and programs will be a tiny niche market for those unable to break free of MS's iron grip

I use Linux (Gentoo), and I've been using various distros on and off since 2008, and solid since 2011 or so. I usually bounce around between Ubuntu, Debian, and Gentoo. I've messed with Tiny Core and such too. I don't like Ubuntu because of the commercial-ness, Debian because packages are built on developers' machines, and Tiny Core kept breaking (5sec boot was nice though). I'm currently on Gentoo because I can optimize it for my AMD CPU instead of an Intel (with -march=k8 - yes, I have a K8).

Finally, why do I use Linux? I've never heard of this reason before, but it's that most of my favorite software is Linux-only. The cross-platform stuff is easier to build on Linux too, so that I can have "bleeding-edge" (and beyond) stuff. For example, I'm using a version of Firefox (23) that I compiled just last night.